Torque wrenches have been traditionally used to tighten bolts to a desired level of tension. However, a certain amount of torque is required to overcome friction forces and that amount of torque does not get translated into bolt tension. The friction force which usually must be overcome is bolt head friction between the bolt head and surfaces beneath the head. Another influence is thread friction which is the amount of friction between the mating threads of the bolt and a nut.
As a result, the actual tension in the bolt does not generally correlate to the torque applied by the wrench. In order to apply the desired level of tension to the bolt, the amount of torque applied to the bolt must compensate for friction forces. Although lubricants can be applied to the bolt to reduce friction, the amount of torque required to overcome friction remains unknown. Therefore, it is common for bolts to be either over-torqued or under-torqued, which results in the failure of the bolt or in the bolt becoming loose.
The actual amount of torque required to produce a certain level of bolt tension can be determined with a load cell bolt transducer. Generally, a load cell bolt transducer consists of one or more sensing elements which measures torque and tension on the bolt. A single sensing element could be a cylindrical element onto which a series of strain gauges are bonded. The sensing element measures the tension of the bolt as well as thread torque applied to the threads. Thread torque is the amount of torque required to overcome thread friction and to perform mechanical work. By determining the relationship between the separately measured applied torque and the actual tension of the bolt, the correct amount of applied torque for obtaining a certain bolt tension level can be determined.